Skywave is the propagation of electromagnetic waves bent (refracted) back to the Earth's surface by the ionosphere. Radio propagation is a term used to explain how Radio waves behave when they are Transmitted, or are propagated from one point on the Earth Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station (or during sporadic e season, a low band TV station) can sometimes be heard as clearly as local stations. For the band see Broadcast (band Broadcasting is the distribution of audio and/or Video signals which transmit Shortwave Radio operates between the frequencies of 3000 KHz (3 Sporadic E or Es is an unusual form of Radio propagation utilizing characteristics of the earth's Ionosphere. A television station is a type of broadcast station that broadcasts both audio and Video to Television receivers in a particular area Most long-distance HF radio communication (between 3 and 30 MHz) is a result of skywave propagation. High frequency (HF radio frequencies are between 3 and 30 MHz. Since the early 1920s Amateur radio operators, limited to lower transmitter power than commercial radio, have taken advantage of skywave for long distance or DX communication. Amateur radio, often called ham radio, is both a Hobby and a service in which participants called "hams" use various types of Radio communications DXing is the hobby of tuning in and identifying distant Radio or Television signals or making two way radio contact with distant stations in Amateur radio

Role of the ionosphere

The ionosphere is a region of the upper atmosphere, where neutral air is ionized by solar photons and cosmic rays. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on When radio waves reach the ionosphere at a shallow angle, they are partly reflected by the surface. The ionosphere can also be similar to a prism refracting light; different frequencies are "bent" by different amounts. Refraction is the change in direction of a Wave due to a change in its Speed.

Much as the surface of the ocean interacts with the wind, the condition of the ionosphere is constantly changing due to interaction with incoming radiation. When signals have "bounced" off this irregular surface, they may fade in and out and have the "phasing", "flanging" or "fluttery" character familiar to listeners of shortwave music broadcasts.

Depending on the transmitting antenna, signals below approximately 10 MHz during the day and 5 MHz at night may reach the ionosphere at a steep angle (vertical incidence) and be reflected almost straight back down to Earth. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into Alternately, if the antenna "aims" the signal close to the horizon; the signal reaches the ionosphere at a shallow angle and returns to Earth at a medium to long distance.

The Earth's surface (ground or water) reflects the incoming wave back toward the ionosphere again. As a result, like a rock "skipping" across water, the wave may actually "bounce" or "skip" between the earth and ionosphere two or more times. This phenomenon is known as multihop propagation. Signals of only a few watts can sometimes be received many thousands of miles away as a result.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

Other considerations

VHF signals with frequencies above about 30 MHz usually penetrate the ionosphere and are not returned to the Earth's surface. Very high frequency (VHF is the Radio frequency range from 30 MHz to 300 MHz. E-skip is a notable exception, where VHF signals including FM broadcast and VHF TV signals are frequently reflected to the Earth during late Spring and early Summer. TV DX and FM DX are two terms customarily grouped together that refer to long-distance reception of TV and FM Radio stations respectively E-skip rarely affects UHF frequencies, except for very rare occurrences below 500 MHz.

Frequencies below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), propagate most efficiently by skywave at night. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Medium Wave (MW is a part of the Medium frequency (MF radio band used mainly for AM broadcasting. Shortwave Radio operates between the frequencies of 3000 KHz (3 The longwave Radio band is a range of frequencies used for AM broadcasting, which extends from 148 Frequencies above 10 MHz (wavelengths shorter than 30 meters) typically propagate most efficiently during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The Maximum usable frequency for skywave propagation is strongly influenced by sunspot number. Maximum usable frequency (MUF describes in Radio transmission, using reflection from the regular ionized layers of the Ionosphere, the upper A sunspot is a region on the Sun 's surface ( Photosphere) that is marked by intense magnetic activity which inhibits Convection, forming

Skywave propagation is usually degraded -- sometimes seriously -- during geomagnetic storms. A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. Skywave propagation on the sunlit side of the Earth can be entirely disrupted during sudden ionospheric disturbances. A sudden ionospheric disturbance (SID is an abnormally high ionization/ plasma density in the D region of the Ionosphere caused by a Solar

Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface ofthe Earth at night. The Kennelly-Heaviside layer, also known as the E region or simply the Heaviside layer, is a layer of ionised Gas occurring at 90&ndash150 km The ionosphere is the uppermost part of the atmosphere, distinguished because it is Ionized by solar radiation This leads to an increase in the "skip" or "hop" distance of the skywave at night.

See also

External links

• Radio Netherlands: Long Distance MW Listening: Propagation
• Skywaves: Long Distance (DX) radio and television reception. Also medium wave DX, propagation, aerials, etc. for the hobbyist.
• Navy - Propagation of Waves
• NASA - Types of Propagation
• EHamNet propagation page
• Radio wave propagation basics
• Solar Terrestrial Activity Report
• HFRadio Propagation forums
• Rare gamma-ray flare disturbed ionosphere
• Articles on sporadic E and 50 MHz Radio Propagation